Boiling equipment is used mostly in the chemical industry, in the food industry, and in several fields of water economics. The boiling method achieves separation of solutions and of emulsions but claims a high demand on heat. With the boiling method, solutions can be separated into their components. Heat transfer will cause the solution solvent to evaporate leaving a more concentrated solution.
Boiling equipment and its mode of operation can be classified in two different ways. According to one classification there is "evaporation" and "steaming out" boiling equipment. The "evaporation" boiling equipment involves a method carried out at constant pressure with heat transfer occuring immediately after the evaporation of the solvent. With the "steaming out" boiling method, evaporation of the solvent is performed in two steps: in the first step by heat transfer wherein the solution is heated at a given pressure, thereafter in the next step, the solution is fed through a choke means into a space having a lower pressure. This pressure decrease simultaneously permits a portion of the solvent to evaporate and reduces the temperature of the solution. The "evaporation" method is also referred to in the literature as the "expansion process" or more prevalently as "flash-evaporation". The multistage evaporation is expressed in the literature as the "multiflash process."
The aim of boiling processes is mostly the recovery of dissolved material although solvent recovery is also frequently practiced. An example of the latter, for instance, is the desalinization of sea-water by boiling. With this process partly potable water, partly salt is recovered from sea-water.
A major problem with boiling processes is Sedimentation. Crystals derived from the boiler stones are delivered by and sedimented onto the wall of the equipment. This results in narrowing of passages within the vaporization column thereby impairing heat transfer effects. For this reason boiling equipment is preferably constructed in a way that permits cleaning to be carried out as easy as possible. The surfaces to be cleaned should, above all, be easily reachable to insure economic viability of the process.
A further major problem is the very high heat consumption of boiling processes in general and the evaporation and steaming methods in particular. In order to decrease the high heat consumption, recourse may be had to so-called multistage boiling. Theoretically, heat consumption decreases inversely with the number of stages. Practically speaking, however, the finite heat transferring surfaces decrease heat consumption to a lesser extent than that theoretically calculated.
Nonetheless, the multistage boiling approach has recently been incorporated into evaporation type boilers and achieved some success in decreasing heat losses. Again, practically speaking, this means that as many boilers are needed as there are number of stages. All this results in a decrease of heat consumption, but at a multifold increase in equipment capital costs.
Research has recently been undertaken to decrease these capital costs. One widely practiced solution has been to build stage evaporation boiling equipment in a single unit. Much of the expensive control means can thereby be eliminated. In the course of the new research, so called sandwich-units have been prepared as replacements for the single unit bodies. These sandwich units have been fixed by screw-locking into a single apparatus. Instead of regulating devices, fixed choke-means have been incorporated therein. The aforesaid approach has only superficially, but not in actuality, transformed the multistage evaporation boilers into a single unit. Capital costs of this approach are still very high. Moreover, this type equipment spawned other considerable problems.
One of these new problems was imperfect sealing caused by the need to screw-lock a number of different stages together. Even if all other segments of the equipment perform perfectly, seals as is well known will inevitably result in leakages, especially where there are a great number of seals. These leakages can easily ruin the operating economics of boiling equipment, especially that involving a vacuum. With multistage evaporation boiling methods at least part of the stages function under vacuum.
A new problem specific to the aforesaid equipment arises from the fact that plates instead of tubes are used because of space limitations as the heat transferring surfaces. Several draw-backs result from the use of plates. Though the heat transfer proportions and mechanical cleaning possibilities of the plates are more favorable than that of the tubes, plate construction itself is more complicated. Moreover, notwithstanding that mechanical cleaning of the plate equipment is easier, there is still a problem in trying to clean only small segments of the equipment. One can't simply dismantle a small area. Rather, it is necessary to disjoin large segments of the equipment. This is a long and costly procedure. Safety is also compromised by being forced to disconnect a large number of sealing parts.
A final problem should be mentioned. The choke-means, being used to replace the control means, are accessible only by taking apart the equipment. Separate choke-rows are necessary for vapor and liquid. Accordingly, replacing both choke-rows requires care to insure proper positioning. To achieve proper positioning it is the practice to have at least one row of the choke-means preformed in a special changeable construction for some of the units.